1. Field of the Invention
The present invention relates to an aircraft and to a lightning protection fastener for fastening structural members of an aircraft.
2. Description of the Related Art
A wing partly constituting an airframe of an aircraft generally has a hollow structure, and the inner space of the wing is also used as a fuel tank. An outer panel (wing surface panel) forming a wing surface is fixed to a support member for supporting the backside of the outer panel with a fastener (fastening member).
In most cases, outer panels are made of carbon fiber reinforced plastics (composite material), which has light weight, high strength, and improved durability. Use of outer panels made of carbon fiber reinforced plastics, which has low conductivity, causes lightning current at the time of lightning strikes to concentrate to metallic fasteners. This tends to cause a high current exceeding an allowable current level to flow along interfaces between elements including fasteners, outer panels, and support members, thereby allowing an electric discharge to occur in the vicinity of those interfaces. The electric discharge abruptly raises the temperature of the elements such as fasteners and support members, which can cause parts of those elements to be molten and scattered.
In order for an aircraft to have a sufficient explosion-proof performance at the time of lightning strikes, it is necessary to keep the discharge spark and scattered molten substances away from fuel vapor to prevent the occurrence of flames. One possible method for achieving this is to cover fasteners with an insulating member. This can prevent lightning current from flowing to fasteners, thereby preventing the occurrence of electric discharges.
U.S. Unexamined Patent Application, Publication No. 2010/0224724 teaches a fastener with an insulating member of an insulating plastic material attached to its head portion located on the outer surface side of an outer panel. The head portion of the fastener has a substantially frustoconical shape with its diameter increasing with distance from a shaft portion of the fastener, and is accommodated within a space defined by a receiving surface formed in the outer panel. The insulating member is engaged in the outer periphery of the head portion of the fastener, and has a seat surface formed such that it is flush with the seat surface of the fastener when the insulating member is engaged to the fastener.
An outer panel made of fiber reinforced plastics has a lower stiffness than a metallic fastener. FIG. 7 schematically illustrates an outer panel 100, a support member 110, and a fastener 80. When the fastener 80 is tightened, a receiving surface 101 of the outer panel 100 is pressed by a seat surface 81 of the fastener 80 and slightly depressed. An insulating member 90 has a seat surface 91 formed to be flush with the seat surface 81. When the receiving surface 101 is depressed, a shaft-side portion 91A of the seat surface 91 is pressed hard against the receiving surface 101 of the outer panel 100 by a convex engagement portion (click) 82 of the fastener 80. On the other hand, an outer-surface side portion 91B of the seat surface 91 is warped upward relative to a center portion 92 of the insulating member 90 by friction against the receiving surface 101. While an airframe of an aircraft requires high smoothness so as to lower air resistance as much as possible, the above mentioned deformation of the insulating member 90 makes it difficult to attain the desired smoothness.
In view of the above technical problem, it is an object of the present invention to provide required smoothness of an airframe of an aircraft that includes an outer panel made of fiber reinforced plastics, and a fastener for fastening the outer panel to a support member where the fastener is equipped with an insulating member.